The inside surface of the faceplate of a field-emission display device (also referred to as a flat panel display) contains pixels. For color displays, each pixel is typically separated into three pixel assemblies, each pixel assembly containing one of three colors (e.g., red, blue or green) of phosphor material. The following discussion can also be applied to monochrome displays where all pixel assemblies have the same color of phosphor (including white) rather than pixel assemblies of different colors. The technique described below can be applied to plasma, cathode ray tube and other display devices as well as field-emission display devices.
For the case of a color field-emission display device, electrons are directed from an electron emitter into each pixel assembly to excite the phosphor therein and cause it to emit light. Light generated in this fashion travels either toward the viewer through the faceplate or away from the viewer. A thin coating of reflective material, typically aluminum, can be layered across the rear surfaces of the pixel assemblies to reflect light toward the viewer. The reflective layer can also function as an anode to attract the electrons emitted by the electron emitters. When used, the reflective layer is relatively thin, on the order of 300-500 Angstroms, to enable electrons to pass from the electron source to the phosphor material without losing a significant amount of energy.
The pixel assemblies are typically separated into rows and columns by an opaque mesh-like structure commonly referred to as a black matrix. The black matrix functions to increase the contrast of the display by sharply demarcating a pixel assembly of one color from a pixel assembly of another color and by absorbing ambient light. In addition, by separating pixel assemblies, a three-dimensional black matrix prevents electrons directed at one pixel assembly from being "back-scattered" and striking another pixel assembly, thus helping to maintain a field-emission display device with sharp resolution. The black matrix is also used as a base on which to locate structures such as, for example, support walls. Another important function of the black matrix is to provide a surface to which the reflective layer of aluminum can adhere.
In one embodiment of the prior art field-emission display device, a color filter material is also incorporated in each pixel assembly between the phosphor material and the faceplate in order to enhance the visual display by transmitting only the desired wavelength and absorbing the rest. This color filter may or may not be incorporated in the display, depending on the manufacturing cost and contrast requirements.
With reference to Prior Art FIG. 1, a top view of the inside surface of faceplate 105 of a field-emission display device is illustrated. Black matrix 110 separates the faceplate into a plurality of rows and columns of pixel assemblies 115. A layer each of color filter material and of phosphor material are contained within each of pixel assemblies 115.
With reference to Prior Art FIGS. 2A and 2B, one prior art method of forming a pixel assembly and applying the reflective layer of aluminum to faceplate 105 of a field-emission display device is described. Black matrix 110 and pixel assembly 115 are shown in cross-sectional view. For clarity, a single pixel assembly 115 is shown with two side walls formed by black matrix 110. In actuality, a plurality of pixel assemblies exist, each surrounded on four sides by the black matrix, some sides of which may be taller than others.
With reference first to FIG. 2A, a layer of a selected color (e.g., red, blue or green) of color filter material 220 is deposited into pixel assembly 115. Next, a layer of a selected color of phosphor material 230 is deposited into pixel assembly 115. Then, a layer of lacquer 240 is applied, followed by deposition of the reflective layer.
In one embodiment of the prior art in which a color filter material is used, the selected color of color filter material 220 or phosphor material 230 is spread entirely over all pixel assemblies 115; for example, red phosphor material is spread over all pixel assemblies, including those pixel assemblies in which the red phosphor material is not intended to remain. Then, a photolithographic process is applied in a specific pattern to the rows and columns of pixel assemblies so that only the color filter material or the phosphor material that is intended to remain in the pixel assemblies is exposed to the process; in the example, only the pixel assemblies where the red phosphor material is to remain are exposed to the photolithographic process. The color filter material or the phosphor material exposed to the photolithographic process hardens sufficiently through photo-polymerization while the unexposed material does not. The unexposed material is then washed away, leaving behind only the selected color. This process is repeated for each of the colors of color filter material or phosphor material remaining to be applied until each pixel assembly 115 contains a layer of color filter material 220 and a layer of phosphor material 230.
The prior art method described above is problematic because it results in a significant amount of color filter material and phosphor material being wasted. In general, approximately two-thirds of the color filter material and phosphor material is washed away in each step of the application process. In addition, the prior art process is time-consuming because it employs a number of repetitive steps (e.g., six steps) to apply each color of color filter and phosphor material.
In other prior art methods, processes are employed to selectively apply the different colors of color filter material and phosphor material only into the pixel assemblies where the material is intended to remain. These other prior art methods apply only one color of one material at a time. These other methods include the following: a process in which a dust of the material is applied to a patterned sticky layer, a process in which the material is suspended in an electrolytic bath and an electric field is applied causing the material to be attracted to a patterned glass, a process in which electrostatic fields are used to cause a dry powder of the material to be attracted to a patterned charged substrate, and a process in which the material is screen-printed onto a substrate in a pattern. These alternative prior art methods alleviate the wastage issue; however, they are problematic because they still require multiple process steps, one step for each color, and so remain time-consuming.
Continuing with reference to FIG. 2A, in the next step of the aforementioned prior art method, lacquer material 240 is deposited over the entire surface of phosphor material 230 and black matrix 110. In one prior art method known as the float lacquer process, faceplate 105 is immersed in water. A film of lacquer material is formed on the surface of the water. The water is then drained and the lacquer material settles onto faceplate 105, including phosphor material 230 and black matrix 110, as the water level is reduced. In another prior art method known as the spray lacquer method, water is sprayed over the entire surface of faceplate 105, and then a layer of lacquer material is applied over the entire surface of phosphor material 230 and black matrix 110. As the water evaporates, the lacquer material settles onto faceplate 105.
The individual particles of phosphor material 230 are irregularly shaped. Hence, the water provides a smooth surface over which lacquer material 240 is applied to create a smooth lacquer surface. Thus, when the water is drained or evaporated away, lacquer material 240 in turn forms a smooth surface over the particles of phosphor material 230. Aluminum reflective layer 250 is then deposited over lacquer material 240. The smooth surface of lacquer material 240 results in a mirror finish to reflective layer 250.
With reference now to FIG. 2B, faceplate 105 is exposed to a high temperature (e.g., it is baked in an oven) so that lacquer material 240 evaporates away through pores in reflective layer 250, leaving in the pixel assembly the layers of color filter material 220, phosphor material 230, and reflective layer 250. As indicated in the illustration, reflective layer 250 is also located over the side wall and top surface of black matrix 110.
The prior art is problematic because the lacquer material is applied over the entire surface of the inside surface of the faceplate. Thus, the entire surface area of the layer of lacquer material is exposed to particulates in the surrounding atmosphere before the reflective layer is applied. These particulates settle on the surface of the lacquer material and introduce imperfections into the surface of the reflective layer. For example, particles protruding from the surface of the lacquer material could cause pitting in the reflective layer. The imperfections in the reflective layer diminish the mirror surface of the reflective layer and hence reduce the reflective capability of the mirror surface.
Another disadvantage to the prior art is that the imperfections caused by the particulates in the lacquer material can result in weak spots in the reflective layer. For example, the pitting caused by particles creates areas where the reflective layer is thinner, and these areas may significantly weaken the reflective layer, especially considering the thinness of the reflective layer. During operation of the field-emission display device, the reflective layer is subjected to significant electrostatic loads due to the electrical potential that exists between the electron emitters (i.e., the cathode) and the reflective layer (i.e., the anode). The electrostatic loads exert a pulling force on the reflective layer that can cause it to break or tear apart at weak spots. A tear in the reflective layer reduces the reflective capability of the mirror surface. In addition, a tear in the reflective layer creates stringers of aluminum that induce arcing between the electron emitters and the faceplate. This arcing dims the pixel assembly by damaging it or by reducing the flow of electrons into it, and thus the quality of the display is reduced. If the damage is extensive, it may be necessary to replace the faulty portion of the field-emission display device. This causes added costs to either the manufacturer or the owner of the field-emission display device, and also causes inconvenience and loss of productivity during the period of time when the device is being repaired and is unavailable.
The prior art is also problematic because, as described above, the lacquer material is applied over the black matrix as well as the pixel assemblies. With reference back to FIG. 2A, the lacquer material drapes over the side wall of pixel assembly 115, but it doesn't drape over cleanly, leaving a gap 216 between lacquer material 240 and black matrix 110. Alternatively, lacquer material 240 may thicken in the area (indicated by 217) between phosphor material 230 and black matrix 110. With reference now to FIG. 2B, when the lacquer material is evaporated away, gaps 216 and 217 will be formed between reflective layer 250 and black matrix 110 and between reflective layer 250 and phosphor material 230, respectively. These gaps, also referred to as "tenting," prevent reflective layer 250 from properly adhering to phosphor material 230 and black matrix 110 where the tenting occurs. In addition, the adhesion of reflective layer 250 to the side wall and top surface of black matrix 110 is reduced by the lacquer material applied to those surfaces. Although lacquer material 240 is evaporated away, it initially forms a barrier between black matrix 110 and reflective layer 250 that can reduce adhesion. Without proper adhesion between the reflective layer and the supporting surfaces, the capability of the reflective layer to withstand the pulling forces introduced by the electrostatic loads is reduced and creates weak spots in the reflective layer, especially considering the thinness of the reflective layer. As discussed above, a weak spot in the reflective layer can cause it to tear or break apart, thus reducing the quality of the display.
Thus, a need exists for a method for fabricating a pixel assembly on a faceplate of a field-emission display device wherein the method reduces the wastage and time associated with the application of the color filter and phosphor materials. A need also exists for a method that improves the adhesion of the reflective layer to the black matrix. A further need exists for a method that addresses that need and also reduces or eliminates the imperfections and weak spots introduced in the reflective layer that are associated with the application of the lacquer material.